Technical Field
Embodiments of the subject matter disclosed herein generally relate to generating seismic waves in a non-vertical borehole, and more specifically, to deploying a seismic source that can travel through a non-vertical borehole and achieve a good coupling with the surrounding medium when shooting.
Discussion of the Background
In geophysical prospecting, gas and oil reservoirs are sought by performing seismic surveys of sedimentary rock formations using variations of seismic wave propagation velocity from one layer to another. Reflected, refracted and transmitted waves are detected by seismic receivers after traveling through an explored formation.
Hydrocarbon reservoir surveillance during production is a tool employed to reduce operating costs and maximize recovery of oil and/or gas. Time-lapse (4D) seismic methods use seismic surveys performed during distinct survey periods to monitor changes in the reservoir during production. Seismic velocity and density of a formation, including a producing reservoir, depend on rock type and fluid properties. Changes in seismic responses between surveys may be caused by changes in reservoir saturation, pore fluid pressure during fluid injection or depletion, fractures, temperature changes, etc.
In time-lapse seismic or 4-D seismic, seismic surveys are repeated typically using the same survey acquisition parameters. The time lapsed between consecutive surveys may range from about several months up to a year. After the recorded seismic data sets have been processed to form an image of the surveyed underground, difference plots are produced to highlight where change has occurred within the reservoir. This information is useful for adjusting pumping and/injection schedules, to help manage the development of a hydrocarbon reservoir.
One problem with time-lapse seismic surveys is that soil moisture content, temperature and other factors vary over time. This means that a coupling between a surface source and the seismic receivers can change significantly between surveys. A solution that avoids the pitfalls of the above noted survey is permanent reservoir monitoring, where sources and receivers are permanently installed underground in consolidated material and cemented into rock formations. However, this method is expensive and the buried equipment cannot be reused.
An economic alternative to permanent reservoir monitoring with good repeatability is needed. A method that can be used in sensitive areas where disturbance of the topsoil may be an issue is valuable as well. Accordingly, it is desirable to develop methods and sources that avoid the above-identified drawbacks of conventional methods.